linux-kernel - Re: DMA-API attr - DMA_ATTR_NO_KERNEL

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Message-ID: <c9fe4253-5698-a226-c643-32a21df8520a@arm.com>
Date:   Mon, 1 Jul 2019 15:09:46 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Pankaj Suryawanshi <pankajssuryawanshi@...il.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: DMA-API attr - DMA_ATTR_NO_KERNEL_MAPPING

On 28/06/2019 17:29, Pankaj Suryawanshi wrote:
> On Wed, Jun 26, 2019 at 11:21 PM Christoph Hellwig <hch@...radead.org> wrote:
>>
>> On Wed, Jun 26, 2019 at 10:12:45PM +0530, Pankaj Suryawanshi wrote:
>>> [CC: linux kernel and Vlastimil Babka]
>>
>> The right list is the list for the DMA mapping subsystem, which is
>> iommu@...ts.linux-foundation.org.  I've also added that.
>>
>>>> I am writing driver in which I used DMA_ATTR_NO_KERNEL_MAPPING attribute
>>>> for cma allocation using dma_alloc_attr(), as per kernel docs
>>>> https://www.kernel.org/doc/Documentation/DMA-attributes.txt  buffers
>>>> allocated with this attribute can be only passed to user space by calling
>>>> dma_mmap_attrs().
>>>>
>>>> how can I mapped in kernel space (after dma_alloc_attr with
>>>> DMA_ATTR_NO_KERNEL_MAPPING ) ?
>>
>> You can't.  And that is the whole point of that API.
> 
> 1. We can again mapped in kernel space using dma_remap() api , because
> when we are using  DMA_ATTR_NO_KERNEL_MAPPING for dma_alloc_attr it
> returns the page as virtual address(in case of CMA) so we can mapped
> it again using dma_remap().

No, you really can't. A caller of dma_alloc_attrs(..., 
DMA_ATTR_NO_KERNEL_MAPPING) cannot make any assumptions about the void* 
it returns, other than that it must be handed back to dma_free_attrs() 
later. The implementation is free to ignore the flag and give back a 
virtual mapping anyway. Any driver which depends on how one particular 
implementation on one particular platform happens to behave today is, 
essentially, wrong.

> 2. We can mapped in kernel space using vmap() as used for ion-cma
> https://github.com/torvalds/linux/tree/master/drivers/staging/android/ion
>   as used in function ion_heap_map_kernel().
> 
> Please let me know if i am missing anything.

If you want a kernel mapping, *don't* explicitly request not to have a 
kernel mapping in the first place. It's that simple.

Robin.